1. Field of the Invention
The present invention relates to measurement during drilling and in particular to measurements relative to the behaviour of a drill bit fastened to the end of a drill string.
2. Description of the Prior Art
There are well-known measuring techniques for acquisition of information relative to the dynamic behaviour of drill strings, using a series of bottomhole pickups connected to the surface by an electric conductor. In French Patent Application 92-02,273, two series of measuring pickups connected by a logging type cable are used, one being situated at the well bottom, the other at the top of the drill string. However, the presence of a cable along the drill string interferes with the actual drilling operations.
French Patents 2,645,205 and 2,666,845 describe surface devices placed at the top of the string, which determine certain drilling dysfunctions according to surface measurements, but without taking physically account of the dynamic behaviour of the string and of the drill bit in the well.
Between the bottom of a well and the surface, there is a drill string along which energy-dissipative phenomena (friction on the wall, torsion damping, . . . ), flexibility-conservative phenomena occur, notably under traction-compression. There also is a distortion between bottomhole and surface displacement measurements, which mainly depends on the intrinsic characteristics of the string (length, stiffness, geometry), on the friction characteristics at the pipes/wall interface and on random phenomena.
The information contained in surface measurements is therefore not sufficient to solve the problem of knowing the instantaneous displacements of the bit by knowing the instantaneous displacements of the string at the surface. Surface measurement information must be completed by independent information of a different nature, taking into account the structure of the drill string and the behaviour thereof between the bottom and the surface is a function of a knowledge model which establishes theoretical relations between the bottom and the surface.
The methodology of the present invention uses the combination of an a priori defined model and of surface measurements acquired in real time.
The present invention thus relates to a method of estimating effective longitudinal behaviour of a drill bit fastened to the end of a drill string and driven in rotation in a well by a driving device situated at the surface, using a physical model of the drilling process based on general mechanics equations and wherein the following steps are carried out:
determining parameters of the model by taking into account characteristic parameters of the well and of the string,
reducing the model and retaining only selected natural modes of a state matrix of the model.
According to the method, at least two values Rf and Rwob are calculated in real time, Rf being a function of the principal oscillation frequency of the weight on hook WOH, for example in the zero to ten Hz. range, divided by the average instantaneous rotating speed at the surface, Rwob being a function of the standard deviation of the signal of the weight on bit WOB estimated by a reduced longitudinal model from measurement of a signal of the weight on hook WOH, divided by an average weight on bit WOB0, defined from a weight of the string and an average weight on hook, and any dangerous longitudinal behaviour of the drill bit determined from the values of Rf and Rwob. Rf can be compared with an interval whose bounds are so determined that there is no dangerous longitudinal behaviour of the bit if Rf is not contained in the interval.
Rf can be contained in the interval and a dangerous longitudinal behaviour of the drill bit is quantified according to the values of Rwob.
Rf can be such that       R    1    =            20      *              ∫        WOH                    RPM      0      
where fwoH, expressed in Hertz, is the principal oscillation frequency of the WOH in the zero to ten Hz. range and RPM0, is the average instantaneous rotating speed at the surface, expressed in revolutions per minute.
The bounds of the interval can be 0.95 and 0.99.
In the method,       R    WOB    =            S      wob              WOB      0      
where Swob is a standard deviation of the signal of the weight on bit WOB estimated from that of the weight on hook WOH and from a reduced longitudinal model, WOB0 is the average weight on bit, defined from the mass of the string and from the average weight on the hook.
It can be determined that, for Rwob less than 0.6, there is no danger, and that, for Rwob ranging between 0.6 and 0.8, there is a moderate danger, and for Rwob greater than 0.8, there is significant danger.
The invention also relates to a system for estimating an effective longitudinal behaviour of a drill bit fastened to the end of a drill string driven in rotation in a well by a driving device situated at the surface, wherein a computing unit provides a physical model of the drilling process based on general mechanics equations, parameters of the physical model are identified by taking into account parameters of the well and of the string and the computing unit reduces the model to retain only selected natural modes of a state matrix of the model. The system calculates, in real time, at least two values Rf and Rwob, Rf being a function of the principal oscillation frequency of the weight on hook WOH, for example in the zero to ten Hz. range, divided by the average instantaneous rotating speed at the surface, Rwob being a function of the standard deviation of a signal representing weight on bit WOB estimated by the reduced longitudinal model from measurement of the signal the weight on hook WOH, divided by the average weight on bit WOB0, defined from the weight of the string and the average weight on the hook. The system comprises an alarm relative to any danger of the longitudinal behaviour of the drill bit from the values of Rf and Rwob.
The method and the system can be applied to determination of any danger of bit-bouncing of the drill bit.